One-carbon organic compounds such as methane and methanol are found extensively in nature, and are utilized as carbon sources by bacteria classified as methanotrophs and methylotrophs. Methanotrophic bacteria include species in the genera Methylobacter, Methylomonas, Methylomicrobium, Methylococcus, Methylosinus, Methylocystis, Methylosphaera, Methylocaldum, and Methylocella (Lidstrom, 2006). Methanotrophs possess the enzyme methane monooxygenase, that incorporates an atom of oxygen from O2 into methane, forming methanol. All methanotrophs are obligate one-carbon utilizers, unable to use compounds containing carbon-carbon bonds. Methylotrophs, on the other hand, can also utilize more complex organic compounds, such as organic acids, higher alcohols, sugars, and the like. Thus, methylotrophic bacteria are facultative methylotrophs. Methylotrophic bacteria include species in the genera Methylobacterium, Hyphomicrobium, Methylophilus, Methylobacillus, Methylophaga, Aminobacter, Methylorhabdus, Methylopila, Methylosulfonomonas, Marinosulfonomonas, Paracoccus, Xanthobacter, Ancylobacter (also known as Microcyclus), Thiobacillus, Rhodopseudomonas, Rhodobacter, Acetobacter, Bacillus, Mycobacterium, Arthobacter, and Nocardia (Lidstrom, 2006).
Most methylotrophic bacteria of the genus Methylobacterium are pink-pigmented. They are conventionally referred to as PPFM bacteria, being pink-pigmented facultative methylotrophs. Green (2005, 2006) identified twelve validated species in the genus Methylobacterium, specifically M. aminovorans, M. chloromethanicum, M. dichloromethanicum, M. extorquens, M. fujisawaense, M. mesophilicum, M. organophilum, M. radiotolerans, M. rhodesianum, M. rhodinum, M. thiocyanatum, and M. zatmanii. However, M. nidulans is a nitrogen-fixing Methylobacterium that is not a PPFM (Sy et al., 2001). Methylobacterium are ubiquitous in nature, being found in soil, dust, fresh water, sediments, and leaf surfaces, as well as in industrial and clinical environments (Green, 2006).
The existence of PPFM bacteria as colonizers of the leaf surfaces of most (if not all) species of plants (ranging from algae, mosses and liverworts, and angiosperms and gymnosperms) suggests that PPFM bacteria may play an important role in plant physiology (Corpe and Rheem, 1989; Holland and Polacco, 1994; Holland, 1997; Kutschera, 2007). The fact that plants produce and excrete methanol, probably as a waste product of pectin metabolism in growing plant cell walls, suggested to these researchers that a symbiotic relationship exists, with the PPFM bacteria feeding on the plant-produced methanol and in turn providing positive benefits to the plants. The suggested benefits of PPFM bacteria on plant physiology include positive effects on nitrogen metabolism, seed germination, and stimulation of plant growth through the provision of PPFM-generated cytokinin plant hormones. The use of PPFM bacteria to improve plant growth, plant yield, seed germination, male fertility, and plant nutritional qualities has been disclosed in U.S. Pat. No. 5,512,069, U.S. Pat. No. 5,961,687, U.S. Pat. No. 6,174,837, U.S. Pat. No. 6,329,320, U.S. Pat. No. 7,435,878, and US Patent Application Pub. No. 2006/0228797. In addition, PPFM bacteria have been found to increase the yield of cultivated algae, suggesting their application to the production of algae-derived biofuels (US Patent Application Pub. No. 2011/0269219).